Electronically steerable antenna system utilizing controllable dipolar resonant plasma column



I. KAUFMAN ELECTRONIC-ALLY STEERABLE ANTENNA SYSTEM UTILIZING CONTROLLABLE BIPOLAR RESONANT PLASMA COLUMN Filed July 29, 1963 2 Sheets-Sheet 1 ODULATION D\(7NAL SOURCE EZ L E iEYFl-l \NPUT INPUT 5\6NAL 2 z wzTisOQ E FHZLD VECTOR Z5- FREQUENCY //?l //V6 K4 L/FMA/v I NVENTOR A TTORNE Y BYf March 5, 1968 I. KAUFMAN ELECTRONIC-ALLY STEERABLB ANTENNA SYSTEM UTILIZING CONTROLLABLE DIPOLAR RESONANT PLASMA COLUMN Filed July 29, 1963 Amazo- WAVE GENERATOR 5 500 M C,

R ECEJV ER 2 Sheets-Sheet 2 RECEIVER E I ND\ CATOR MxcRo WAVE GENERATOR //e w/va /(A uF/M A N INVENTOR United States Patent Ohio Filed July 29, M63, Ser. No. 298,174 6 Qlaims. (Cl. 343-754) This invention relates generally to electromagnetic wave systems in which an elongated column of plasma or ionized gas is utilized, and more particularly to antenna systems which include dipole resonant plasma c01- umns, methods of operating such systems, and to electronic arrangements for steering or angularly deflecting the radiation beam formed by such antenna systems when they are used as transmitting antenna or for steering the antenna sensitivity pattern of such an antenna system when it is being used for receiving radio frequency signals.

In copending US. patent application Ser. No. 264,515, filed Mar. 12, 1963, now Patent No. 3,238,531 and assigned to the assignee of the present application, there are disclosed and claimed certain arrangements for utilizing a plasma or ionized gas column as a frequency selective electromagnetic wave antenna.

In accordance with the present invention, elongated plasma columns essentially similar to those mentioned in the above-identified copending application are used in a substantially different manner and employ substantially dilierent operating characteristics to provide for elec tronic steering or beam deflection of transmitting and receiving antennas.

The method and apparatus of the present invention has immediate utility in application to any one of a variety of beam forming antenna systems for transmission, or reception, or both. There is a need in the communications field as well as in the radar field for high gain, narrow beam antennas which provide rapid angular scanning of the direction of the narrow beam. To meet this demand, workers in the antenna art have developed larger antennas and have gone to higher frequencies where atmospheric attenuation and the difiiculty of high power generation prove to be serious limitations. The desire for high gain arises from the need to communicate over increasingly greater distances. The narrow beam requirement stems from the demand for increased precision of direction finding in radar systems and from the demand for power conservation in communication links. A primary difiiculty in achieving the foregoing ends is the necessity in any steerable antenna system of selectively repositioning the antenna beam direction.

In systems where antenna structure are physically rotated or otherwise mechanically repositioned, the mechanical inertia of the structures and the magnitude of the angular accelerations required have presented practical limitations; that is, the desired angular repositioning rates frequently demand unrealizable driving torques and excessive mechanical stresses on the antenna structures and their supporting apparatus. The foregoing practical limitations are a stron incentive for developing systems which embody a stationary antenna structure and electrical arrangements for angularly readjusting the direction of the antenna beam relative to the antenna structure. In the prior art, various electronically scanning antennas have been developed which depend on signal phase shifting to control the direction of the antenna beam. In such systems the high mechanical inertia of a massive antenna structure is, of course, avoided; however, it is replaced by the mechanical inertia or the electrical inductance of the electronic phase shifting mechanisms. Nevertheless, electronic scanning is extremely attractive in that the inductive inertia of an all-electronic system can be overcome by increased voltages rather than mechanical torques and because a greater latitude of beam positioning programs is possible.

In addition, the usual electronically scanning antennas which depend on signal phase shifting networks suffer from the fact that they become quite complex in that they generally require at least one phase shifting element for each radiating element of the antenna. The phase shifting elements must be driven either mechanically or electrically to establish the required phase front for a narrow antenna beam. Usually the phase shifting elements are controlled by a computer network which also serves as a means for indicating the instantaneous position of the antenna beam. In general, mechanical phase shifters are limited to a speed of beam repositioning equal to about five milliseconds per beam width. Likewise, all-electronic phase shifting networks provide a maximum scan rate of about 0.1 millisecond per beam width. In addition, electronic scanning antenna systems frequently have undesirably high side lobes, lower gain and lower beam positioning accuracy than is frequently desired.

Accordingly, it is a primary object of the present invention to provide an improved, narrow beam, electronically steerable antenna system.

It is a different object of the invention to provide a method and apparatus for using a dipole resonant plasma column as an antenna for transmitting or receiving electromagnetic wave energy.

It is another object of the invention to provide an improved antenna system and scanning method employing resonance characteristics and plasma guide wave propagation characteristics of elongated plasma columns.

It is a further object to provide an improved antenna apparatus for selectively and individually receiving a plurality of electromagnetic wave signals which jointly occupy a common frequency band or have the same frequency and different directions of wave propagation.

It is a still further object of the invention to provide an improved electrically steerable antenna which is tunable over a band of frequencies in the microwave region and which enables low-loss translation of signals within a selected relatively narrow frequency band together with substantially complete exclusion of signals having frequencies outside that selected band.

In a preferred form of the present invention, the aforementioned difficulties and limitations of the prior art are overcome and the foregoing objects are achieved by utilizing the known phenomenon of dipolar resonance in an elongated plasma column and by utilizing the phenomenon of propagation of electromagnetic waves longitudinally along such a column when the free electron density of the plasma is higher than that which corresponds to plasma resonance at the frequency of the electromagnetic waves which are to be received or transmitted. In accordance with one exemplary embodiment of the present invention, a plasma column enclosed in an insulating envelope is positioned to extend transversely through a region subjected to microwave radiation with the relative orientation being such that the electric field vector of the radiation is substantially perpendicular to the longitudinal axis of the plasma column. When so arranged, and with the plasma column energized by a discharge current passing longitudinally therethrough, transverse electric fields of the microwave radiation produce oscillatory transverse displacement of the electron cloud of the plasma relative to the comparatively stationary ion cloud. Substantially all the electrons in a given elemental cross-sectional portion of the plasma column move transversely to the axis of the column in a coherent or common time phase manner. At the instant of time when the electron cloud has maximum transverse displacement relative to the ion cloud, energy is stored by that elemental cross-sectional portion of the plasma in the form of electrostatic field potential both internally and externally of the plasma. At another instant of time when the electron cloud is minimumly displaced relative to the ion cloud, the electrons have a maximum transverse velocity representing a maximum oscillatory kinetic energy. The natural fre quency of the transverse electronic oscillation is dependent upon a number of physical factors including primarily the free electron density or degree of ionization of the plasma. Accordingly, the natural resonant frequency of the plasma may be tuned over a wide frequency range by controlling the magnitude of the discharge current pass ing through the plasma. Thus the resonant frequency of the plasma column may be adjusted to the frequency of a communication signal which is desired to be received or to the frequency of the magnetron or other microwave oscillator in the case of a system wherein a plasma column is used as a transmitting antenna. Where the plasma column functions as a receiving antenna a signal utilization circuit is coupled to the oscillating near electric field of the plasma in a manner such that the circuit is substantially non-responsive to the primary radiation to which the region is subjected. By that arrangement, the apparatus enables reception of selected signals which are within the narrow band pass characteristic of the resonant plasma column and provides for substantially complete rejection of extraneous signals such as wide band noise, jamming and the like. The various forms of apparatus within the scope of the present invention are all dependent for their operation on the principle of plasma resonance or dipolar resonance of a column of ionized gas plasma.

It can be shown that a column of plasma located away from dielectrics or conductors acts as an electrical resonator having a resonant frequency w w A/i. The basic concept and theory of dipolar resonance in a plasma is not, per se, a new discovery but has been considered by a number of workers in the art. For example, in an article entitled Plasma Resonance in Ionospheric Irregularities, Arkiv f. Fysik, 1951, vol. 3, page 247, by N. Herlofson it was shown that a plasma column suspended in free space and illuminated by electromagnetic energy would produce energy reflection and absorption at certain frequencies dependent on the plasma density. That phenomenon was of interest in the investigation of radar reflections from meteor trails. It was shown that a long column of ionized gas could produce a much larger reflection when excited by a wave whose electric vector was transverse to the column rather than by one whose electric vector was parallel. Thus, while the phenomenon of plasma resonance has been investigated in the prior art, applicant is unaware of any prior art teaching the suggestion of practical application of the phenomenon to antenna apparatus or the like.

In another field of prior art there has been a fairly complete investigation of the use of gas discharge phenomena in microwave systems. In this branch of the prior art the gas discharge has generally been utilized for functions and purposes not requiring plasma resonance. A fairly complete survey of this particular branch of the prior art appears in an article entitled, Microwave Applications of Gas Discharges by F. R. Arams, Electronics, November 1954, pp. 168172. In that article it is noted that ionized gas plasmas have been suggested as microwave noise sources, t-r and air switch devices, attenuators, phase shifters, electronic switches, tunable resonant cavities and non-reciprocal gyrators. Moreover, the Electronics article seems to recognize that a plasma characteristically has a critical value of the electron density N at which the electrons will oscillate in phase with an exciting electromagnetic field, and suggests the possibility of using such oscillation as a mechanism for bunching or velocity modulating an electron beam.

The prior art as exemplified by the above mentioned Electronics article, has completely failed to appreciate that a quantity of plasma can serve as a high frequency resonator of extended dimensions similar to a line of dipole elements or that such a resonator could be excited at a single local region to produce oscillation over a much larger portion of the plasma. That general concept is utiliZed in each of the hereinafter described embodiments of apparatus in accordance with our present invention. None of the various devices disclosed by the prior art employ transverse or dipolar resonant oscillation of a plasma column to provide an extended line source of radiation in response to excitation of the plasma by a localized field.

As described to this point, the apparatus of the present invention is substantially the same as that described in the above mentioned copending application Scr. No. 264,- 515. The present invention differs from that of the above mentioned application in that the plasma density in the elongated column is not held constant at the optimum density for plasma resonance at the frequency of the radiation which is to be transmitted or received. Rather, in accordance with the present invention, the discharge current applied longitudinally through the plasma column is varied or amplitude modulated pursuant to any desired programming, to cause the plasma density to vary across a range which encompasses and exceeds the optimum density for plasma resonance at the applied frequency. In accordance with the preferred embodiment of the present invention such modulation of the plasma density is accomplished by coupling a source of modulating voltage in series with the plasma column across a high potential direct current source.

The foregoing and other objects and features of this invention will be apparent from the following description taken with the accompanying drawings, through which like reference characters indicate like parts, which drawings form a part of this application and in which:

FIGURE 1 is a simplified diagrammatic illustration of one embodiment of an antenna system in accordance with the invention;

FIGURE 2 is a cross section taken along lines 2-2 of FIGURE 1;

FIGURE 3 is a graph of the frequency response characteristic of the apparatus illustrated in FIGURES l and 2;

FIGURE 4 is a diagrammatic illustration of an arrangement in accordance with the invention utilized as a receiving antenna; and

FIGURE 5 is a diagrammatic illustration of another embodiment of the present invention operating in the transmitting antenna mode.

In FIGURE 1, there is shown in diagrammatic form one example of a microwave antenna system in accordance with the present invention. This system will be discussed in terms of its use as a transmitting system. However, it should be understood that the system is reciprocal; that is, it can be used with equal effectiveness either as a transmitting or receiving antenna or in the case of a radar apparatus as both. Specifically, microwave signals are applied to the apparatus of FIGURE 1 by way of coaxial waveguide 20 which includes a center conductor 22 and an outer conductor 23 which is connected to a conductive reflector plate 12 as indicated by the numeral 24. The conductive reflector 12, which may be a solid or perforated metal plate or a conductive screen, has a central aperture which supports an insulative bushing 18 through which the center conductor 22 of the coaxial waveguide projects. An elongated gas discharge device 10 which constitutes a means for providing a plasma column adjacent the plate 12 is positioned with its longitudinal axis about one-quarter wavelength, at the frequency to be transmitted, from the front surface 14 of the conductive reflector. Preferably the outer end of the center conductor 22 extends closely adjacent one side of the discharge tube It) to a position at least slightly past the longitudinal axis of the discharge tube so that microwave electric fields generated along the center conductor 22 will be efiiciently coupled to the plasma column contained within the discharge tube. The plasma column responds to energy fed thereto by way of the center conductor 22 and tends to oscillate aat the frequency of the applied waves to thereby reradiate the microwave energy and provide a beam or antenna pattern 21. As will be described in further detail hereinafter, the plasma column contained within the discharge tube of FIGURE 1 oscillates at its natural dipolar resonant frequency and provides the antenna pattern 21 only when the free electron density of the plasma is adjusted to a value precisely corresponding to the plasma resonance at the frequency of the applied microwave electric fields. When the plasma density has any other value within a substantial range encompassing and exceeding the density for natural resonance theplasma is forced to oscillate at the frequency of the applied electric fields even though that frequency is slightly different from the natural plasma resonance frequency correspondin to the existing plasma density. When the density of the plasma exceeds that for natural resonance at the frequency of the applied electric fields electromagnetic waves are propagated along the plasma column. That is, the cross-sectional portion of the plasma which is immediately adjacent the probe 22 oscillates precisely in phase with the x fields applied thereto from the probe 22. Successive longitudinally spaced crosssectional portions of the plasma column oscillate at the same fre uency but with a substantial phase displacement corresponding to the speed of propagation of the electromagnetic waves along the surface of the column.

The plasma column contained within the discharge device it) may be considered as a linear array of dipole antenna elements all of which are fed with energizing signal of the same frequency but with the energy fed to the different successive dipoles being progressively phaseshifted. From elementary antenna theory it will be recognized that when a linear array of dipole elements are driven at progressively larger phase angles relative to a reference dipole element, the linear array of dipole elements generates a deflected or angularly shifted radiation beam as illustrated at 25 in FIGURE 1. Thus by varying the plasma density or number of free electrons per unit volume within the discharge tube 10, it has been found that the primary lobe of the antenna pattern can be deflected or steered from the position indicated by the numeral 21 to the position indicated at 25. In order to cylically scan the antenna beam pattern between the two indicated positions the apparatus of FIGURE 1 is provided with an arrangement for cyclical variation or modulation of the plasma density. The density modulation means preferably comprises a modulation signal source which applies a modulation voltage through a transformer 17 to the cathode end of the discharge device 10. The arrangement illustrated for modulating the longitudinal discharge current passing through the discharge device ltl is exemplary only. It will be appreciated that any one of a large variety of common circuit arrangements might be used for cyclically varying or otherwise modulating the plasma discharge current and therefore modulating the free electron density.

In a preferred embodiment of the present invention I have used a mercury vapor discharge tube about 45 centi meters long and having an inside diameter of about 0.7 centimeters. One such discharge tube for providing an elongated plasma column has a cylindrical quartz envelope, a thyratron-type cathode, and a starting electrode located near the cathode (not shown). It should be understood, however, that various other types of discharge tubes may be utilized within the scope of the present invention to provide the results hereinafter noted. For example, it is possible and reasonably practical to use cylindrical Pyrex discharge tubes having an outside diameter of about 11 mm., an inside diameter of about 9 mm., and enclosing a mercury vapor at about 0.1 mm. pressure. With such a tube, dipolar resonance of the plasma column can be obtained at a frequency of about 3350 me. with a longitudinal plasma discharge current of about 1 ampere. It has been found that for best operation, the plasma column provided by the discharge tube 10 should be long in relation to its diameter, a preferable length to diameter ratio being about 29. Since the plasma column operates as a linear array of dipole elements it will be appreciated that the width of the antenna beam lobe will depend upon the physical length of the array. For example, to obtain a very narrow beam pattern, the plasma column preferably should be several wavelengths long at the chosen operating frequency. While the present invention is not restricted to any particular form of discharge device or the above noted relative dimensions, it will be apparent that the use of the shortest practical discharge device which fits the requirements of the particular application will have the immediate advantage of a lower voltage drop during operation.

Full appreciation of the present invention in all its aspects requires a brief consideration of the principles of plasma resonance. Electronic oscillation in a volume of ionized gas plasma was recognized and reported as early as 1931 by L. Tonks in an article entitled, Plasma- Electron Resonance, Plasma Resonance and Plasma Shape, in Phys. Rev. 1931, vol. 38, pp. 1219-1223, and has since been further investigated by others. It has been demonstrated heretofore that a cylindrical plasma column suspended in free space, when illuminated by a beam of electromagnetic wave energy having its electric field vector substantially normal to the column, will produce wave energy reflection and absorption at certain frequencies dependent upon the plasma density. The resonant plasma response to the incoming E-field is essentially a coherent oscillation of the plasma electrons in a direction parallel to the E-field and transverse to the plasma column. Physical visualization of the plasma electronic oscillation is illustrated in FIGURE 2 as transverse oscillatory displacement of an electron cloud 30 oscillating in the horizontal direction relative to the comparatively stationary ion cloud 32. FIGURE 2, of course, represents an elementary cross section of the plasma column which is contained within the discharge tube 22 of FIGURE 1.

The electric fields produced externally of the plasma column are the same as those of a line of electric dipoles. Thus, the plasma behavior appropriately can be regarded as dipole resonance. In oscillations at resonance, the stored energy in the system of the plasma column oscillates between the forms of electrostatic energy of surface charges and of kinetic energy of the transversely moving plasma electrons.

It has been determined that a very long plasma column in free space will exhibit dipole resonance at a resonant frequency given by as wherein o is the angular frequency of an impinging electromagnetic wave, o is the plasma angular frequency equal to 5.6 10 /N, and wherein N is the free electron density (electrons/cm?) While other modes of resonance, such as a quadruple mode, are also possible, such modes of resonance of higher angular order are usually not excited by an impinging electromagnetic wave. They will not be considered any further herein.

Returning to the dipole mode, it has been found that dipole resonance is also possible if the plasma column is surrounded by a dielectric tube. The relation corresponding to (1) is now in 1rd m 1rd mvra 1. WT hi) "1:1, 2, and

I 1 K K are modified Bessel functions of the fiI'Si and second kind, respectively.

Although, by Equation 3, the resonant frequencies differ for the various values of m, it is found that for a ratio of L/a equal to or greater than 20 (as used in various systems which have been constructed), the lower order modes coalesce to the resonance given by Equation 1.

The plasma column may be excited into resonance by an impinging electromagnetic wave of the proper frequency and polarization, or by a coupling mechanism to the near field. An example of the latter is the near field probe 22, shown located near the plasma column in FIGURE 1.

When the apparatus of FIGURE 1 operates as a receiving antenna, incoming electromagnetic waves as designated by the numeral 28 in FIGURE 2 cut across the discharge tube and thereby induce transverse oscillatory movement of the electron cloud 3! relative to the ion cloud 32. As described heretofore, the transverse oscillatory movement of the electron cloud produces an external near electric field as indicated by the field lines 34. This oscillatory electric field cuts across the near field probe 22 and induces therein a microwave signal corresponding in frequency and amplitude to the oscillation of the plasma column. The signal thus developed in the probe 22 is coupled directly to the coaxial waveguide and may be applied therefrom to an appropriate receiver or other signal utilization load means (not shown).

If the frequency of the incoming microwave radiation 23 as shown in FIGURE 2 is not closely related to the plasma density in the discharge tube 10 in a manner to satisfy Equation 1, dipolar resonance of the plasma is not excited, no signal will be generated in the probe 22 and no power will be coupled to the coaxial line 20. On the other hand, if the plasma density is related to at least one frequency component of the incoming radiation 28 in the manner specified by Equation 1, that particular frequency component will excite the plasma column and the column will therefore couple power to the waveguide 20. It has been found that when the plasma density is only slightly higher than the density required to satisfy Equation 1 at the frequency of the incoming microwave radiation, the plasma column will oscillate at the frequency of the applied radiation but the oscillations at successive longitudinally spaced cross-sectional portions of the plasma column will be phase-shifted to an extent dependent upon the departure of the plasma density from that density which would optimumly correspond to plasma resonance at the frequency of the applied radiation in accordance with Equation 1. The present invention utilizes that phenomenon by varying or modulating the free electron density so that successive longitudinal cross-sectional portions of the plasma are induced to oscillate at the frequency of the incoming radiation and at relative phase relations depending upon the angle of the incoming .vave front relative to the longitudinal axis of the discharge device 16'. That is, as illustrated in FIGURE 1, if the wave front of incoming radiation is as indicated at 27 and hence substantially perpendicular to the deflected antenna lobe 25 that Wave front will excite cross-sectional portions of the plasma near the right hand end of the discharge device 15* in a manner to cause those plasma portions to lead in phase relative to the plasma. portion immediately adjacent the probe with the amount of lead or phase displacement being proportional to the longitudinal distance of the particular plasma portion from the center conductor 22. Thus the system of FIGURE 1 when used as a receiving antenna responds to incoming radiation with such radiation causing transverse oscillation at different phase angles all along the length of the plasma column. The differently phased oscillations in the plasma column result in propagation of travelling waves longitudinally along the plasma column. The velocity of propagation of such waves is dependent upon the free electron density of the plasma. When the density at a particular instant in the density modulation cycle has a value corresponding to that required to provide the antenna lobe indicated by the numeral 25 the waves propagated along the discharge device 18 reenforce one another to create a maximum near electric field extending from the oscillatory plasma in the region immediately adjacent the probe 22. That electric field generated in the plasma in response to the incoming radiation is coupled to the probe 2.2 and therefrom to the coaxial conductor fat, from which it may be applied to a conventional receiver or other signal utilization means. By variable or cyclically controlling the longitudinel discharge current, applied to the discharge tube from the direct current source 13+, the plasma density N may be varied in accordance with any desired beam scanning program and the direction of maximum sensitivity of the plasma column antenna is thereby stored or deflected between the positions indiated by the lobes 21 and 25.

FIGURE 3 illustrates the band pass characteristic of the antenna system of FIGURE 1. In FIGURE 3 frequency is plotted as the abscissa and the ordinate axis represents the power output in decibles relative to an arbitrary input power level. On curve 36 of FIGURE 3, points 37 and 38 indicate the half-power points or the frequency at which the output power is down 3 decibels from the input power level. With a center frequency of about 3,540 me. as indicated in FIGURE 3, the antenna of FIGURE 1 provides a bandwidth of about me. between the half-power points; that is, if the microwave radiation impinging on the antenna systern of FIGURE 1 includes frequency components between about 3,460 me. and 3,620 mc., those components Will be received and translated by way of coaxial waveguide 26 to the receiver circuitry while noise signals, jamming signals, or other undesired signals below 3,460 me. or above 3,620 me. will be comparatively attenuated by the antenna system. The relatively narrow bandpass characteristic of the antenna apparatus of the present invention is particularly advantageous in pulsed radar systems where it is frequently desirable to provide maximum rejection of noise and jamming signals.

In FIGURE 4 there is illustrated another embodiment in accordance with the present invention wherein the signal received by the plasma column antenna is coupled to a receiver or signal utilization load means 56 by way of an arrangement including a rectangular waveguide 48. In this embodiment the waveguide 43 serves substantially the same general function as the coaxial line 20 of the apparatus illustrated in FIGURE 1. Specifically, in FIGURE 4 the antenna structure comprising the discharge tube It) enclosing the ionized gas plasma column 4-0 serves as the receiving antenna of a communication system. The communication system additionally may include a transmitter which is illustrated diagrammatically as comprising a microwave generator 42 from which power is coupled to a transmitting horn 44 and is thereby radiated generally toward the plasma column 44) with the transmitted radiation being oriented so that the electric field vector 46 is substantially perpendicular to the direction of propagation and perpendicular to the longitudinal axis of the plasma column. The radiation impinging on the plasma column causes transverse dipolar oscillation in the plasma in the same manner as described heretofore in connection with the apparatus of FIGURE 1. The near electric fields generated by the oscillating plasma cut across a pickup probe 54 which is supported by an insulative bushing 52 in one side wall of the waveguide 48 and is positioned transversely adjacent the envelope of the discharge tube 10. Voltage generated in the probe 54 by the near electric fields of the plasma column are coupled along the probe 54 to the interior of the waveguide and generate waves therein for propa gation downwardly along the waveguide and by way of any conventional coupling arrangement to a receiver 56. The receiver 56, of course, may comprise a travelling wave tube amplifier and a conventional crystal detector or any of various other wellltnown arrangements for utilizing small amplitude microwave signals. To provide maximum coupling from the plasma column to the waveguide, the waveguide 48 is provided at its upper end with a conventional tuning plunger or piston 50 for adjusting or tuning the waveguide for maximum response to the resonant frequency of the plasma column.

As described heretofore in connection with FIGURE 1, the plasma column of FIGURE 4 also operates as a steerable antenna. That is, it will receive signals in a selective manner from any one of a number of transmitters positioned at different angles relative to the axis of the discharge device 10 and generally in the plane which is perpendicular to the paper and includes the transmitting horn 44 and the axis of the discharge device 10.

The embodiment of FIGURE 4 may be operated with the input wave energy being propagated from the transmitting horn 44 to the plasma column 4%. However, since the plasma column is a reciprocally operative element it will be understood that the apparatus can operate reversely, with the microwave power to be transmitted being applied from an appropriate generator through the waveguide 48 and being coupled to and radiated by the plasma column 40.

In FIGURE 5 there is illustrated afurther embodiment in accordance with the invention which is similar to the arrangement just mentioned in that the plasma column is utilized as part of the transmitting antenna. Specifically, a microwave generator 42' feeds microwave energy to the input end of a parallel wire transmission line 64. Coupling from the generator 42 to the input end of the transmission line 64 may be had by any of various conventional means such as, for example, a conventional Balun coupler. Accordingly, such coupling is designated diagrammatically by the numeral 652. From its input end the parallel wire transmission line 64 extends through an aperture in a metallic reflector plate 66 and straddles the elongated discharge device In a preferred embodiment, the axis of the discharge device It) is spaced one-quarter wavelength from the front surface of the reflector 66 and the parallel wire transmission line 64 extends beyond the axis of the discharge device 10 a distance of one-half wavelength at the resonant frequency so that a voltage maximum along the transmission line occurs approximately at the discharge device 10, thereby providing maximum coupling of microwave energy from the transmission line 64 to the plasma column. The use of the parallel wire transmission line 64 has the advantages that direction radiation from the parallel wire line is considerably less than that which should be radiated from the single setup 54 of the arrangement illustrated in FIGURE 4. Accordingly, when the parallel wire line is used, the plasma column itself is the principal radiator and a negligible amount of energy is radiated directly by the transmission line 64. In addition, the parallel wire transmission line arrangement has the advantages that it provides a balanced system for driving the plasma column and prevents distortion of the antenna pattern of the oscillating plasma column. As shown in FIGURE 5, microwave energy at the plasma resonant frequency is radiated by the plasma column in a narrow beam directed toward a receiving system comprising horn 68, a conventional receiver 56, and an indicating means 57 coupled to the output of the receiver. Thus the apparatus of FIGURE 5, considered as a Whole, constitutes a communication system in which the resonant plasma column forms the essential element of a highly directional steerable transmitting antenna.

The above mentioned copending application Ser. No. 264,515 discloses in complete detail an electromagnetic wave antenna system utilizin a plasma column which operatively constitutes a linear array of dipole elements when excited by radiation of a frequency having a specified relation to the free electron density of the plasma in the column. In the various systems there disclosed, it has been the usual practice to energize the column so that each elemental cross-sectional portion of the column has a plasma density corresponding with reasonable precision to the density value required for plasma resonance at the frequency of the applied radiation. By so energizing the plasma column all of the dipolar resonant cross-sectional portions of the column have been caused to oscillate inphase transversely of the column. With precise in-phase oscillation of the elemental dipolar plasma portions, the various systems taught by application Ser. No. 264,515 all produce a beam pattern or antenna sensitivity pattern having its primary lobe almost exactly perpendicular to the longitudinal axis of the plasma column 1.0 as shown in FIGURE 1.

The present invention differs from the teachings of the above mentioned application in that a method and apparatus are provided for causing the plasma density to depart from the optimum density value corresponding to plasma resonance at the frequency of the applied radiation.

It has been discovered that when the plasma density, in an apparatus such as that illustrated in FIGURE 1, exceeds the theoretically optimum density value for plasma resonance, the plasma will oscillate in the transverse dipolar mode at the frequency of the impinging radiation, but the antenna lobe or sensitivity pattern provided by the linear array of dipole elemental plasma portions will not remain normal to the plasma column. Rather, the antenna beam pattern is deflected angularly to an extent dependent upon the departure of the plasma density from the optimum resonance value. In accordance with the method and apparatus of the present invention electronic deflection or steering of the antenna beam pattern is accomplished by programming variations in the longitudinal discharge current through the plasma. As the discharge current is varied, either periodically or aperiodically de pending upon the application, the plasma density is correspondingly varied through a range of densities which includes and substantially exceeds the resonance density value dictated by Equation 1, supra. In the specific embodiment illustrated in FIGURE 1, steering of the antenna beam between the positions indicated by the numerals 2 1 and 25 is accomplished by a discharge current modulator means 15 connected serially with the discharge device it) across a current source 13+. Of course, various other circuit arrangements for cyclically or aperiodically varying the plasma discharge current may be used and are considered to be within the scope of the present invention.

In one system which has been constructed to utilize the present invention the discharge current pas 'ng through a mercury vapor discha ge tube of 9.0 mm. i side diameter has been cyclically varied from 0.1 to ap roximately 1.5 ampercs. With that degree of cyclical amplitude modulation of the current it was found that the peak of the principal lobe of the antenna pattern was periodically scanned -rom 0 (normal to the discharge tube axis) to angle of about 60 relative to the normal or perpendicular position. it is believed that the critical current range for angular beam scanning of apparatus having the above parameters is about 1.1 to 1.3 amperes.

It is to be noted that the present invention distinguishes from prior art electronically steerable antennas in that such prior art systems generally require either modulation of the frequency of the waves transmitted or modulation of the relative phases of a number of driving signals. The present invention requires only a single cobrstant frequency source of radio frequency energy.

While applicant does not wish to be restricted to any particular theory or explanation for the beam deflection or steering function of the method and systems in accordance with this invention, it is presently believed that such beam deflection results from generation of plasma guide waves of radio fre ency energy travelling along the discharge tube M from the exciting element 22. Similarly, such waves travellin" in opposite directions can give rise to an antenna which radiates from a standing wave pattern or a combination of the same with a travelling wave. It can be shown that when the plasma density of a column exceeds the optimum value for resonance in accordance with Equation 1, applied electromagnetic energy will propagate along the column. The relative phases of oscillation of successive spaced elemental portions of the plasma will depend upon the velocity of such propagation, i.e. upon the wavelength in the plasma of the propa gating surface waves. That wavelength apparently is variable as a function of the density of the plasma. Thus, the relative oscillatory phases of the spaced dipolar elemental portions of the linear antenna array are variable as a function of the plasma density and, therefore, the angular position of the principal antenna lobe is controllable and variable in response to variation of the current which is passed longitudinally through the gas discharge device.

While the present invention has been described with reference to certain specific embodiments only, it will be obvious to those skilled in the art that it is not so limited but is susceptible of various changes and modifications without departing from the spirit and scope thereof.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An electronically steerable microwave antenna comprising:

a plasma column which exhibits transverse oscillation of the electron cloud of the plasma relative to the ion cloud at a dependent variable resonant frequency wherein:

N is the independently variable number of free electrons per cubic centimeter in said plasma, and

K is the composite effective dielectric constant of said plasma column;

excitation means including a source of microwave energy of a predetermined frequency for applying microwave radiation to said column with the electric field vector of said radiation polarized substantially normal to the axis of said plasma column; and

means including a current source connected to pass current longitudinally through said plasma column for varying the free electron density of said plasma through range of densities which includes and eX- ceeds the value of N corresponding to plasma resonance at the frequency of said microwave energy source so that said plasma. column exhibits longitudinal surface wave propagation during time intervals when the free electron density exceeds the density corresponding to plasma resonance at the frequency of said source with such propagation resulting in dipolar oscillation of longitudinally successive portions of said plasma at successively greater phase displacements whereby the antenna beam pattern of the aligned oscillatory plasma portions is scanned angularly relative to the plasma column as the free electron density is varied. 2. in a beam scanning electromagnetic wave antenna for selectively receiving waves from any one of a plurality of angularly spaced wave sources,

an elongated ionized-gas plasma column extending in a direction substantially normal to the electric field vector of said waves and positioned to intercept portions of said wave energy, said plasma column characteristically exhibiting dipole plasma resonance 0scillation at a frequency which varies as a function of the free electron density of the plasma;

means to pass a controllably variable gas discharge current longitudinally through said column for varying said free electron density through a range of densities which includes the particular density corresponding to dipole plasma resonance at the frequency of said waves; so that the antenna beam formed by the plasma column is angularly swept through an angle including at least two of said wave sources with the time function of the beam sweep coresponding to the time function variation of said free electron density.

3. A radio frequency antenna system for selectively receiving electromagnetic wave energy, said system comprising:

a gas discharge device extending substantially normal to the electric field vector of said wave energy for providing, when energized, an elongated plasma column, said plasma column characteristically exhibiting dipole plasma resonance oscillation at a frequency which varies as a function of the free electron density of the plasma and characteristically exhibiting propagation of resonant frequency waves along the plasma column in a manner such that successive clemental portions oscillate in the dipole mode at successively later phase relations;

means including a current source connected to pass discharge current longitudinally through said discharge device for controlling said electron density;

means positioned adjacent said discharge device and responsive substantially only to the dipole electric fields produced by said oscillating plasma for deriving signals therefrom; and

means coupled with said current source for varying said discharge current in accordance with a predetermined function of time to thereby variably control the angular position of maximum sensitivity to impinging electromagnetic wave energy.

4. A microwave antenna for discriminating between a plurality of electromagnetic waves which impinge thereon from different angular directions, comprising:

an elongated plasma column;

means for propagating said waves toward said column with an orientation such that the electric field vector of said waves is substantially normal to the direction of propagation and to the lonigtudinal axis of said column;

means to variably control the free electron density of said plasma for varying the same through a range of densities exceeding that of plasma resonance in said column at the frequency of said waves so that said column exhibits longitudinal surface wave propagation with successive longitudinal portions of the plasma oscillating at different relative phase angles and functioning as differently phased dipole antenna elements;

13 and means for extracting energy from the oscillating dipolar electric fields of said column to produce output signals which are substantially exclusively representative of a selected one of said waves.

5. In a microwave antenna system for discriminating between electromagnetic waves impinging thereon from different directions, the combination of:

means for applying said waves to an elongated column of ionized-gas plasma with the oscillating electric fields of said waves oriented approximately normal to the longitudinal axis of said column;

plasma density control means for varying the density of free electrons in said plasma through a range which includes and exceeds that density which satisfies the relation wherein:

w =5.6 l /:N m is the angular frequency of said waves, K is the composite effective dielectric constant of said column of plasma, and N is the free electron density of said plasma expressed in electrons per cubic centimeter, so that said plasma exhibits surface wave propagation therealong and operatively constitutes a linear array of dipolar antenna elements with high frequency electrical energy being dynamically stored in the plasma of each longitudinal elemental portion in the form of oscillations between electrostatic field energy and kinetic energy of transversely moving electrons and with the oscillations in different elemental portions of said column being differently phased so that the antenna system sensitivity pattern varies angularly as a function of the free electron density; and means coupling to the near electric fields of said plasma column for deriving output energy substantially exclusively corresponding to waves impinging from the direction of contemporaneous maximum sensitivity of the antenna system. 6. In a microwave antenna system for selecting one of a plurality of input microwave signals having the same fre quency and emanating from angularly spaced sources:

an elongated column of plasma which characteristically exhibits dipolar resonant oscillation at frequencies dependent upon the free electron density and which exhibits surface wave propagation along the column when the electron density is higher than that which corresponds to dipolar resonance at the frequency of applied electromagnetic waves with such longitudinal propagation resulting in differently phased dipolar oscillations of different longitudinally spaced elemental portions of said plasma so that the plasma column operatively constitutes a linear array of dipolar antenna elements having different oscillatory phases;

ca is the center frequency of a frequency band which encompasses the frequency of said microwave signals,

N is the free electron density of said plasma in electrons per cubic centimeter, and

K is the composite effective dielectric constant of said plasma column;

means for passing said microwave signals across said column with an orientation relative to said column such that the electric field vector of the waves is approximately normal to the longitudinal axis of said column and such that the directions of propagation of said signals intercept said axis at acute angles;

means coupling to the oscillatory near electric field of said column to derive microwave output signals which correspond at any given time to the one of said input signals which intercepts said axis at an angle most nearly corresponding to the maximum sensitivity angle of said array of dipolar antenna elements.

References Cited UNITED STATES PATENTS 2,082,042 6/1937 Wolff 343l 2,142,648 1/1939 Linder 34370l 2,641,702 6/1953 Cohen et a1 343-70-1 3,238,531 3/1966 Kaufman et a1. 343-84O 3,262,118 7/1966 Jones 343701 ELI LI EBERMAN, Primary Examiner. 

3. A RADIO FREQUENCY ANTENNA SYSTEM FOR SELECTIVELY RECEIVING ELECTROMAGNETIC WAVE ENERGY, SAID SYSTEM COMPRISING: A GAS DISCHARGE DEVICE EXTENDING SUBSTANTIALLY NORMAL TO THE ELECTRIC FIELD VECTOR OF SAID WAVE ENERGY FOR PROVIDING, WHEN ENERGIZED, AN ELONGATED PLASMA COLUMN, SAID PLASMA COLUMN CHARACTERISTICALLY EXHIBITING DIPOLE PLASMA RESONANCE OSCILLATION AT A FREQUENCY WHICH VARIES AS A FUNCTION OF THE FREE ELECTRON DENSITY OF THE PLASMA AND CHARACTERISTICALLY EXHIBITING PROPAGATION OF RESONANT FREQUENCY WAVES ALONG THE PLASMA COLUMN IN A MANNER SUCH THAT SUCCESSIVE ELEMENTAL PORTIONS OSCILLATE IN THE DIPOLE MODE AT SUCCESSIVELY LATER PHASE RELATIONS; MEANS INCLUDING A CURRENT SOURCE CONNECTED TO PASS DISCHARGE CURRENT LONGITUDINALLY THROUGH SAID DISCHARGE DEVICE FOR CONTROLLING SAID ELECTRON DENSITY; MEANS POSITIONED ADJACENT SAID DISCHARGE DEVICE AND RESPONSIVE SUBSTANTIALLY ONLY TO THE DIPOLE ELECTRIC FIELDS PRODUCED BY SAID OSCILLATING PLASMA FOR DERIVING SIGNALS THEREFROM; AND MEANS COUPLED WITH SAID CURRENT SOURCE FOR VARYING SAID DISCHARGE CURRENT IN ACCORDANCE WITH A PREDETERMINED FUNCTION OF TIME TO THEREBY VARIABLY CONTROL THE ANGULAR POSITION OF MAXIMUM SENSITIVITY TO IMPINGING ELECTROMAGNETIC WAVE ENERGY. 